Here's the actual cost to ship a bass and case weighing 15 lb. from my Twin Cities zip code to commercial address in an Atlanta zip code, in a 52x21x7 box (the box my SG Faded with hard case came in):
UPS 45.64
Fedex 39.04
USPS 34.01
In all three cases, the weight is upped based on the size of the box, e.g. UPS charges a 47 lb. rate for this size box.
Smallest bass box I have is 51x16x5, that's for a short scale bass with a gig bag. That would still be 27.25 UPS to a commercial address in Atlanta, charged as 25 lbs. It would still be 34.01 by USPS.
Im not even remotely concerned with the cost of shipping a bass at the moment...but you picked Atlanta as your proposed location and I haven't been back to Minnesota in a couple years so Ill pick a few other items to fill your proposed box with and we can test the rates.
Ill start with a 16x24 souffle cake from the Wuollet bakery, some overpriced Tomato soup from Byerlys, a large Cariboo coffee, a taco wrap from Maynards on lake Minnetonka, a pair of Juicy Lucys from Matts and a watered down cocktail from Nyes Polonaise Room. If there's any room in the box after that just fill the rest with some crumpled up Star Tribunes or menus from The Gay 90s. Ah, home sweet home.
That should roughly approximate the weight of a 20/20. Head to the nearest Fedex Store and send it down, post a copy of the receipt and we can settle this dispute once and for all.